1. Field of the Invention
The present invention relates to an optronic heading deviation measurement system providing high discrimination of infrared light sources not only from the spatial but also the spectral points of view. By light sources is meant the useful source to be detected as well as parasite sources, in particular the sun; these sources differ by their size, their intensity, their spectral energy distribution and this sets difficulties for discrimination and detection.
2. Description of the Prior Art
It is known to obtain spatial selection of sources by inserting in the optical path one or more modulation or reticles grids formed of slits, or by means of transparencies in the case of optical tracks. The modulation is produced by the movement at uniform speed of the grid in a direction orthogonal to the strip for modulating the signal at the frequency of the slits, or according to the corresponding code, the electronic detection being provided by the detector and by matched filtering or downstream correlation circuits. The dimensions of the detector elements and of the transparencies are adapted to the mean dimensions envisaged for the light spot representing the useful target in a predetermined distance range.
To further increase the spatial discrimination it is known to use optical tracks in which the rectilinear profile of the transparencies is modified in accordance with a jagged line forming a herringbone pattern.
To provide spectral discrimination, it is known to use optical filtering with reduced pass band whose central frequency is adapted to that of the radiation of the expected useful target. A solution described in the patent U.S. Pat. No. 4,028,544 uses an assembly of two optical filters, one corresponding to the spectrum of the target to be detected and the other to a spectral band external to this first band; furthermore, the receiving objective is organized for separating the incident radiation into two optical paths and providing focusing at different positions offset by the diameter of the light spot corresponding to the useful target. Each of the optical paths includes one of the filters.
It is also known to provide uniaxial linear scanning of the instantaneous field so as to cover the whole desired extent. The simplest solution consists in using a mirror capable of rotating about an axis; more complex solutions use rings of mirrors or dihedrons.